The Possibility Of
...This [trend] made many scholars conjecture that maintaining the bird of imagination and rendering it obedient are among those matters that are beyond the realm of possibility and adjacent to natural impossibilities. However, this is not the case, for it is possible to make it obedient by means of spiritual mortification and training and spending time, to the extent that the bird of imagination is in his hold and does not move except by his will and choice...
-Imam Ruhullah Khumayni
The Possibility of Concentration In Prayer
Almighty Allah says in the Holy Qur'an:
Allah does not charge a soul
Save to its capacity. 
The stereotype that concentration in prayer is impossible is one of the greatest hurdles that hamper man from wayfaring towards the Divine proximity and attaining his perfection. It is indeed, according to some, a Satanic ruse inculcated to hinder man from attaining his felicity and happiness in both the worlds.
The human being, therefore, should be extremely cautious before accepting any unestablished notion without convincing proof and evidence. He should never be deceived with statistical data and inductive reasoning to conclude that huduru'l qalb (the presence of heart) is impossible or next to impossible.
Akin to physical training that requires continuous exercise and practice, mind control too can be achieved with time and struggle. Man should realize that his faculty of imagination is habituated to flit to different places all the time. Therefore he should strive to curb this habit and bridle it so that it is directed by his will.
The late Imam Khumayni says:
And of the faculties that accept orientation and training are the faculties of imagination (khayal) and estimate (wahm), for surely, before orientation, they resemble a bird that frequently escapes and perpetually moves; it flies from one branch to another and moves from one thing to another, such that, if the human being was to evaluate the movements it has made in one minute's time, he would find that it has moved continuously to (different) things on extremely trivial instances and irrelevant occasions. This [trend] made many scholars conjecture that maintaining the bird of imagination and rendering it obedient are among those matters that are beyond the realm of possibility and adjacent to natural impossibilities. However, this is not the case, for it is possible to make it obedient by means of spiritual mortification and training and spending time, to the extent that the bird of imagination is in his hold and does not move except by his will and choice, and (thus) he suppresses it whenever he wants to, wherever he wishes to, and in whatever instance he chooses to do so- such that it is held in the location (said instance) for hours. 
Therefore judging the statistical data acquired from a group of the masses should not quickly lead one to make conclusions of its impossibility. Only unwavering evidence should be sought to find out the truth. Such evidence may be gotten from the Holy Qur'an, the Sunna, or impartial rational judgments.
Evidence From the Qur'an And the Holy Narrations
The verse that calls the believer to keep up prayer for the purpose of Allah's remembrance (20:14)  as well as the traditions mentioned in the previous section also clearly prove the possibility of concentration in prayer: how can the Wise Creator command man to keep up prayer to remember Him when the latter cannot concentrate therein?
The traditions also informed us that the degree of acceptance of the daily prayer depended on the extent of one's attention therein.
Can such directions of guidance be taught to the believer if they are impossible to achieve? Obviously Allah does not burden any soul save what it can bear to handle. The Holy Qur'an says:
Allah does not charge a soul save to its capacity. 
Hence one cannot deny the possibility of attaining concentration in prayer. The task being difficult cannot be taken as a reason of its impossibility. If man struggles and is orderly in his venture, he would achieve his objective. The Holy Qur'an says:
And that a man shall have to his account only as he has labored. 
1. Allah does not enjoin that which the human being cannot bear
2. Allah has enjoined us to pray for His Remembrance
Therefore, His Remembrance is a possible phenomenon.
3. And Remembrance without attention is no Remembrance
Therefore attention in prayer is a possible phenomenon.
 Holy Qur'an, 2:286
 Imam Ruhullah al-Khumayni, al-Adab al Ma'nawiyyah Lissalat, pp. 95-96
 'Keep up prayer for remembering Me.'
 Holy Qur'an, 2:286
 Ibid., 53:39